Gold Microstructure Sample Preparation Process
Prior to analyzing the microstructure with a metallographic microscope, the preparation of a metallographic specimen for the metal workpiece is required.
The preparation process for metallographic specimens typically consists of the following five steps:
Sample collection:
Samples should be of a size that is easy to hold and grind, typically a cylinder with dimensions Φ15mm x 15-20mm in length, or a cube with edge lengths of 15-25mm. For samples with special shapes or small sizes that are difficult to hold, inlay or mechanical clamping is required.
2. Embroidery Sample
The two types of inlaying include cold inlay and hot inlay, with materials such as phenolic powder and mica powder. Phenolic powder is opaque, comes in various colors, is relatively hard, and the sample is not easy to be chamfered; however, its corrosion resistance is poor. Mica powder is semi-transparent or transparent, with good corrosion resistance but softer. Both materials require specialized inlaying machines for pressure and heat application to form the inlay.
Materials sensitive to temperature and pressure, such as quenched martensite and soft metals prone to plastic deformation, as well as specimens with microcracks, should be cold-set and washed, then can be cured at room temperature without altering the specimen's microstructure. Epoxy resin and dental stone embedding methods are particularly suitable for powder metals and ceramic porous specimens.
3. Polishing:
Coarse grinding: Flatten the specimen and grind it into the appropriate shape, usually on a grinding wheel machine.
Polishing: Often performed on sandpaper. Sandpaper is categorized into water-soluble and metallographic sandpaper. Water-soluble sandpaper typically uses SiC abrasive, which is not soluble in water, while metallographic sandpaper features abrasives such as artificial corundum, silicon carbide, and iron oxide, which are harder and have multi-angled edges, offering good cutting properties. During precision polishing, water can be used as a lubricant for either manual wet sanding or mechanical wet sanding.
Generally, polishing can be performed after using grits of 240, 320, 400, 500, and 600 sandpaper for smoothing. For softer metals, finer metallographic sandpaper should be used for smoothing before polishing.
Polishing:
Transform fine polishing scratches into a glossy, scratch-free mirror finish.
Coarse grinding: The commonly used abrasives for coarse grinding, which involves removing the polished deformation layer, are α-Al2O3, Cr2O3, or Fe2O3 with particle sizes of 10-20μm, mixed with water to form a suspension for use. Currently, synthetic diamond abrasives have gradually replaced abrasives like alumina.
Superfinishing (also known as final polishing): Removes the deformation layer produced by rough polishing, minimizing polishing damage. Requires the operator to possess advanced skills.
Caution: During the polishing process, select appropriate additives based on the material type to avoid chemical reactions caused by improper additives. For instance, aluminum materials should never be polished with alumina polishing powder, as it can lead to chemical reactions, altering the material's tissue structure and thereby affecting test data and results.
5. Microstructure Sample Chemical Etching
Clean the polished specimen thoroughly with water or wipe off the surface residue with alcohol. Then, immerse the specimen surface in the corrosive agent or use bamboo or wooden clamps to hold a cotton ball soaked in the corrosive agent to wipe the surface. The polished surface will gradually lose its luster. Once the specimen has reached the appropriate level of corrosion, rinse it clean with water immediately.
To observe the sample surface under a microscope, simply dry it with filter paper or a hairdryer. For high magnification, the corrosion should be slightly less pronounced, while for low magnification, the corrosion should be deeper.
